its because a few customers have hot water outside taps,seems a good idea on a cold morning
Might be worth mentioning if the water you use is too hot in cold temps you run the risk of thermal shock cracking glass.
I myself use an onboard hot water heater for wfp work so can turn the heat down when working in cold temps. However unfortunately using a customer hot outside tap/di on demand setup you have no way of controlling the temp of the purified water coming out of your brush so best to be aware of this.
As far as tds readings are concerned the tds reading remains the same if I'm using hot, warm or cold purified water. However after saying that I have no idea if resin would be effected by running hot water through a di tank I as I've never had the need to find out the way my system is set up. I can only assume hot water would have an impact on resin as I was advise that the heater should be connected just before the hose reel.